New troubleshooting/networking article covering the three SSH failure modes
after a fleet migration (stale hardcoded IP, Tailscale 1.98.x cold-path
teardown, rebuilt-box host-key mismatch) and the durable fix (MagicDNS names +
known_hosts purge + ConnectTimeout), with the WSL2 no-resolver caveat.
Cross-links the existing host-key article (adds a 'when pinning the IP is
wrong' callout) and adds the SUMMARY nav entry.
